Having developed wing-warping for lateral control, the Wrights now addressed control in pitch, or climb and descent. To control climb and descent, the pilot must be able to control the movement of the center of pressure.

Lift and the center of pressure

Lift is the vertical force acting on a wing. The focal point of this lift force is called the center of pressure. In flight, as the wing changes its orientation to the oncoming flow of air, the center of pressure moves back and forth along the surface of the wing.

Elevator

Otto Lilienthal tried to keep up with the center of pressure’s continual movement by constantly shifting his body weight, which adjusted the center of gravity. The Wrights felt were impractical and dangerous. Instead, they controlled the movement of the center of pressure aerodynamically, by mounting a movable horizontal surface, called an elevator, in front of the wings. Pressure on the elevator would counteract the upward or downward pitching of the airplane due to the changing position of the center of pressure.

Canard configuration

An elevator mounted in front of the wings is known as a canard configuration. A canard lessens the violent reaction that generally occurs when an aircraft with a rear-mounted elevator stalls, or loses lift. This type of stall cost Otto Lilienthal his life. With a canard, the aircraft settles more gently after a stall, a characteristic that saved the lives of Wilbur and Orville on several occasions.

Wing profile

Designing the shape of the wing profile, or airfoil, was also important. Others had already determined that curved wings generated more lift than flat ones. Most had used a perfect arc, with the high point of the curve in the middle. The Wrights placed the high point of the curve much closer to the wing’s leading edge and made the depth of curvature fairly shallow. They believed this would reduce the movement of the center of pressure, making the aircraft more stable and easier to control.

Aerodynamics: Aircraft size

Beyond control and airfoil shape, another consideration was size. How large a wing area was needed, and how light did the glider have to be to lift a pilot into the air? The mathematical relationships between speed, wing surface area, lift, and drag were already well established. With these the Wrights were able to calculate the size, weight, and speed requirements for their glider.

Calculating lift and drag

In 1792 another British engineer, Samuel Vince, showed that the force a fluid exerts on a surface also depends on the surface’s angle to the oncoming flow. Therefore, when calculating the forces of lift and drag acting on a wing, other multiplying factors must be applied: the coefficients of lift and drag. The equations used to calculate lift and drag available to the Wrights are still basically the same ones used today:

Otto Lilienthal compiled and published a table of coefficients of lift and drag for the airfoil shape he used on his gliders. The table became quite well known and was the starting point for aerodynamic research for many experimenters, including the Wright brothers.

Smeaton's Coefficient

18th-century British engineer John Smeaton established that the amount of pressure on the object depends on how fast the fluid moves over it. Following the Smeaton relationship, the air pressure on a wing depends on how fast the air is moving over the wing. When measuring pressure in a flow, the density of the fluid involved—air in this case—must be accounted for with a multiplying factor called a coefficient. A value for air of 0.005 was derived in the mid-18th century and named after Smeaton.